Blown tubular film extrusion

Fig. 1.9 A schematic representation of blown tubular film extrusion. (Reproduced by permission of Oxford University Press.)...

In Europe and other parts of the world the films are produced by a different technique, blown tubular film extrusion. This gives more biaxial orientation to the films which are consequently less prone to splitting. 

Blown film is one of the most common methods of film manufacture (also referred to as tubular film extrusion). The process involves extrusion of a plastic through a circular die, followed by bubble-like expansion. The principal advantages of manufacturing film by this process include the ability to ... 

Blown, or tubular, film extrusion is one of the major processes used for manufacturing plastic films. In this process, plastic pellets are fed into the hopper and melted in the extruder. After exiting the extruder barrel, the molten resin enters an annular die. The resin is forced around a mandrel inside the die, shaped into a sleeve, and extruded through the round die opening in the form of a rather thick-walled tube. The molten tube is then expanded into a bubble of the desired diameter and correspondingly lower film thickness (gauge) by the pressure of internal air, which is introduced through the center of the mandrel. Inside the bubble, air is... 

According to the end use application, PEs are processed by various techniques, which include injection moulding, blow moulding, rotomoulding, and film extrusion. However, since the bulk of the processed material is used as film in the area of packaging, the discussion in this chapter focuses mainly on processing behavior and the ultimate properties of tubular blown film. 

This is a further extension of the extrusion process. A tubular film emerging from a circular die is internally pressurized with air. This extends the film which, together with the temperature and draw off rate, decides the material gauge and degree of orientation. The film may be blown upwards, downwards, or horizontally (Fig. 7.4). 

Sheet material, i.e. material thicker than about 0.25 mm, is usually produced by using a slit-shaped die, whereas thinner material is often produced by a blown film extrusion process in which an annular die is used and air is blown into the centre of the tubular extrudate to blow it into a sort of bubble. At a certain distance from the die the polymer is sufficiently cool to solidify into a film, which is then flattened and collected on rollers. Figure 1.9 illustrates a blown-film system. 

Blown film extnistion and slit die extrusion are the basic methods used to manufacture plastic films. Blown film extrusion produces tubular film and slit die extrusion produces flat film. Tubular film may be cut to produce flat film. 

Figure 1.7 Blown Film Extrusion of Tubular Film [Briston, 1989]...

Blown Film Linos. A blown film line is quite different from a flat film line. In a blown film line, a tubular film is extruded vertically upwards as shown in Figure 28. Air is introduced to the inside of the tube as a result, the tube expands to a bubble with a diameter larger than the diameter of the die. The ratio of the bubble diameter to the die diameter is called the blow-up ratio. T5q>ical blow-up ratios used in LDPE film extrusion for packaging are in the range of 2.0-2.5 l. 

An important development in the extrusion of film is the so called co-extrusion technique, where two or more layers of film are extruded simultaneously to produce a laminate with improved barrier, appearance or mechanical properties. Each layer is fed from a separate extruder. Both cast and blown film techniques are used the composite tubular film may be rotated in the same way as in simple lay extrusion whilst the flat film die allows individual adjustment of the melt streams. 

Han C, Kwack T (1983) Rheology-processing-property relationships in tubular blown film extrusion. J Appl Polym Sci 28 3399-3418... 

Vinex. [Air Prods.] Thennoplastic polyvinyl alcohol cqpOlymer lesin for extrusion, iiy. or blow molding, tubular blown film, bottles, fiber for industrial or personal care applies. 

An extrusion system used extensively for thin films of polyethylene and polypropylene is the blown film process. In this process the polymer is extruded as a relatively thick tubular extrusion and is then blown up by internal air pressure to form a thin-walled tube. The tube is then sUt to form a sheet of film up to 10 ft in width or is left as a flattened tube known commercially as lay-fiat tubing. To improve gauge uniformity in this process, the circular extrusion die, the air cooling ring, or both are frequently rotated. Although low-density polyethylene and polypropylene are the materials most used for this process, polyvinyl chloride, polyamides (nylon), and some polystyrenes have all been used. (See also blown film lay-flat tubing.)... 

Coextruded films are produced by a tubular-blown film process and a flat-die, chill-roll casting process. Capital and operating costs for blown-film vs cast-film coextrusion lines are strongly dependent on product mix and utilization. Equipment suppliers provide comparative economic evaluations for specific products. Practical cast-film equipment has been discussed previously (3). Coextrusion dies are unique. Extruders used before the die and take-away equipment used afterwards are standard equipment for single-layer film manufacture of blown or cast film (see Extrusion). 

When the extrudate exits the die, it has to be quenched and possibly sized (drawn through a fixture) to maintain its final shape. Depending on the extrusion process, different methods are available to quench the final product. Cast film and sheet are quenched on rolls and in water baths, blown film is quenched by air in a blown film tower. Solid profiles, pipe, and tubing are quenched in calibration tanks filled with water and in some cases connected to a vacuum system. Polymer strands and monofilaments are quenched in air or water baths while wire coating is done horizontally in air or water. In small and large part blow molding, the melt is quenched in molds as the extruded tubular parison is inflated. 

Figure 19.4 Industrial smart blender for manufacture of blown film and other tubular extrusions. Because the smart blender substitutes for conventional dies and allows on-line control of structure in an extrusion, it is also referred to as a smart die. ...

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